It is well known to construct a photodetector with a charge coupled device (CCD's). These types of devices are also known as charge coupled imagers (CCI's).
A CCI is constructed from arrays of transparent MOS capacitors. A bias voltage applied to a gate electrode of the MOS capacitor can be used to alter the electric potential within a depleted semiconductor substrate, directly beneath the gate electrode. Appropriate bias levels to the array of MOS capacitor structures allows the formation of "wells" and "barriers" which are used to define picture elements called pixels. When the imager is placed in the focal plane of an optical imaging system, for example, photo-generated charge can be collected in the pixels, reproducing the image in a manner analogous to exposure of film grains in photographic film. The size of the integrated charge packet associated with each picture element, or pixel, is proportional to the incident photon flux.
The use of "barriers" and "wells" is also used to transfer the charge packets within the device. At the end of an exposure cycle, the charge packets defining the electronic image is transferred out of the imaging section and to a charge sensing circuit via a shift register. This shift register is generally formed by a series of adjacent MOS capacitor structures, being termed a CCD shift register.
The detection of the signal charge within the shift register is an important aspect of the operation of CCD's. Techniques for detecting a signal charge are well known in the art and include the use of an external pre-amplifier connected to an output diode, on-chip signal charge sense and reset MOSFET's to form a gated signal charge integrator. The floating gate signal sense MOSFET measures the signal charge packet non-destructively. A full description of these techniques, including references to technical related papers, can be found in "Charge Transfer Devices" by C. H. Sequin and M. F. Tompsett (Academic Press, 1975).
Output circuits can also be used with CCD's. The circuits eliminate reset noise on the output node by correlating double sampling. An extensive description of this technique can be found in "Characterization of surface Channel CCD Image Arrays at Low Light Levels", M. H. White, et al., IEEE J. Solid State Circuits, V. Sc-9, pp. 1-13, 1974.
In a normal mode of operation, a CCD image sensor can produce a video output signal. CCD's can also perform signal processing functions, such as signal differencing. Signal differencing can produce an output voltage signal of the imager which is proportional to the difference of the signal of two adjacent pixels. In a CCD digital filter application, a differential amplifier can be used to perform signal differencing. See for example pages 201 to 235 of "Charge Transfer Devices" by C. H. Sequin and M. F. Tompsett (Academic Press, 1975).
In U.S. Pat. No. 4,639,678, a method and apparatus is described which performs a charge difference of two charge packet signals. In this device, two unknown charge packets are stored in adjacent potential wells of equal depth in a charge coupled device. The charge packets are then merged by changing the potential on an intermediate merge electrode to remove a potential barrier between the two wells. The potential barrier is then reestablished, and a current is induced through one of the electrodes which establishes the two wells of equal depth. The current is integrated as a measure of the original absolute difference between the two charge packets.
The prior art differencing techniques suffers from several problems. First, the techniques are unable to provide the difference between two consecutive charge packets in sequence. Second, the devices are unable to perform the differencing techniques at the relatively high clock rate of the CCD shift register. Third, the prior devices are unable to perform charge differencing and at the same time operate in a non-differencing mode for normal video output.